The present invention relates in general to the art of earth boring and more particularly to a system for retaining the cutting elements in the cutter member body of an earth boring apparatus.
Cutting element life and efficiency are of prime importance in boring holes in the earth. For example, cutting element life and efficiency are important in drilling oil and gas wells and boring tunnels and raise holes. In general, the penetration rate is directly related to the condition of the cutter member and the condition of the cutter member is related to the condition and orientation of the cutting elements.
Cutter members having carbide insert cutting elements located in the body of the cutter member are generally utilized because of the ability of the carbide insert cutting elements to penetrate hard formations. The carbide inserts are mounted in a relatively soft metal forming the body of the cutter member. The most commonly used method of securing the inserts in the cutter member body is to provide cylindrical sockets in the cutter member body, to mold the inserts into a cylindrical shape, and to pressfit the inserts in the sockets in the cutter member body. The inserts are retained in the cutter member body by "hoop" compression generated when the insert is pressed into the relatively soft cutter member body. A frequent cause of short cutting element life and related physical damage to cones, arms and other cutter members is the loss of carbide insert cutting elements, generally designated as compacts, from their physical position in the cone. When these compacts are lost during drilling by the forces related to crushing and breaking rock in the process of drilling the hole, these compacts due to their very hard material properties cause considerable damage to the rest of the cutting structure by becoming enmeshed or wedged between two or more cones. This results in chippage and breaking of other compacts and can result in serious damage to the bit which must then be removed and replaced. This need for early removal of the bit from the hole and replacing it with a new bit is very costly and time consuming and increases the cost of drilling the hole.
Typically the inserts have been generally cylindrical sections pressed into radial cylindrical sockets in the cutter member. Since the cutter member has a circular cross-section, the interference fit at the lower portion of the socket causes expansion of the socket at the upper portion and loss of fit along the length of the insert. The loss of fit results in the inserts becoming loosened in the cutter member body and premature failure of the cutter member. It is common practice to design the press fit of every compact in every row to a constant and relatively high value of interference which results in large induced stresses throughout the cone. When these stresses add to applied stresses, which occur during normal bit operation, the resultant total stresses cause yielding of the cone material and often result in either loss of compacts or fracture of the cone with subsequent failure of the drill bit. The present invention permits the use of lower initial interference fit for the compacts, yet results in high local retention stresses which promote insert retention of those compacts experiencing maximum load at the bottom of the hole by providing increased friction between compact and hole.